1. Field of the Invention
Fault handling in inverter bridges utilizing Insulated Gate Bipolar Transistors (IGBTs) and similar switching devices, such as inverter bridges utilized with three-phase AC motors.
2. Description of the Related Art
Inverter bridges provide controlled energy used to drive inductive loads. A plurality of switches within the inverter bridge are sequentially switched to vary the inverter bridge's output voltage. Typically, output is a three-phase alternating current voltage. For a general background description of inverter bridge topologies and switching schemes, PCT Publications WO 02/37654 A2 published 10 May 2002 and WO 2004/015851 A2 published 19 Feb. 2004 are incorporated herein by reference.
In conventional inverter bridges, fault conditions (short-circuits) can be characterized as falling into one of two categories. A first category is a Type I fault. Typically, a Type I fault is the result of a fault inside the drive (for example, an IGBT failure or an output fault with low inductance). As a result of the low impedance, the increase in current (di/dt) through the IGBT is high, such that the IGBT is forced to desaturate.
When a Type I fault is detected, the switches of the inverter bridge are sequentially commanded to an off-state. Because the switches are not saturated, the current decays slowly, causing no appreciable voltage spikes. Therefore, Type I faults for two-level inverter bridges are fairly easy to manage, and numerous methods are known in the art. However, in three-or-more (multiple) level inverter bridges, which have more than two IGBT switches involved in the fault current path, the desaturation sequence of IGBT switches becomes unpredictable, which may cause IGBT destruction.
A second category is a Type II fault, which is typically a fault external to an inverter bridge, such as a cable short-circuit or motor fault. The impedance in the circuit undergoing fault is typically high, resulting in the current through the IGBT rising slowly. The result is that the IGBT saturates under the Type II fault condition. A Type II fault may occur while a particular IGBT is already in saturation.
Under a Type II fault condition, the IGBT can be quickly turned off, when the IGBT is commanded to an off-state while in saturation. However, such switching while in saturation results in a voltage spike that may destroy the transistor. The voltage spike is due to a counter-electromotive force produced by the negative current di/dt times a leakage inductance of the circuit. Under a Type II fault, the currents rise to levels several times higher than the rated nominal current of the IGBT.
A Type II short-circuit fault is the worst case for an inverter bridge, particularly in a multi-level inverter bridge, because it allows the IGBT to saturate during a fault condition. Type II faults involve a short-circuit path inductance which causes a limited di/dt current increase, which will allow the current to rise well above the maximum rated current of the IGBT, but will not allow the IGBT to enter desaturation. Adding additional circuitry to an inverter bridge to cut off power to the inductive load, without commanding off the IGBTs, as a practical matter, increases the leakage inductance of the inverter bridge circuit, thereby causing otherwise innocuous current switching di/dts to generate harmful voltage spikes. In comparison, if the switches are commanded off during a Type II fault and the fault current rises above the maximum rated current of the IGBT, then the IGBT will most likely be destroyed by the voltage spike generated by the negative current di/dt.
As shown in FIG. 1, at the initiation (t1) of a Type II fault, current rises slowly. However, the time between an inverter controller recognizing an overload situation (t2) and when the short circuit current exceeds a maximum rated current (t3) is often less time than an individual IGBT requires to switch off. Therefore, the fault current enters a forbidden region for switch shut-off before the controller can act, or before a switch commanded to shut-off actually does shut-off.
One way to slow the rise time down further is artificially increase inductance at the output of the inverter bridge. However, this increases losses and does not address the situation when the inductance itself is part of the short-circuit.
Once the fault current enters the forbidden region for switch shut-off, safe shut off is only possible if the current thereafter decreases below the maximum rated current (Imax) of the IGBT, or when the current exceeds the required current to desaturate the IGBT. However, once an IGBT reaches desaturation current levels under a fault condition (t4), there is very little time to turn off the switch before there is thermal damage to the IGBT.
To date, in conventional designs, there is always a fault current inductance that will cause any inverter bridge to risk destruction when commanding the IGBTs to an off-state during a Type II fault. Moreover, with conventional inverter bridges, even when the inverter bridge controller commands switches off in a proper sequence, since desaturation ordinarily occurs randomly among the switches in series, destruction of the switches may nonetheless occur.